unique values for a front matter item

[btrem]

Is it possible to require unique values for a front matter item? Ideally, I'd like it to work the same as output urls, so that if two files have the same value for a front matter item, it'd be the same as if they has the same permalink url.

[pdehaan]

I think I'm a bit confused.
Are you saying you'd like to throw an error if two pages have the same arbitrary matching front matter value?

For example,

---
# src/pages/one.njk
title: Page One
key: home
---
<h1 data-key="{{ key }}">{{ title }}</h1>

Then…

---
# src/pages/two.njk
title: Page Two
key: home
---
<h1 data-key="{{ key }}">{{ title }}</h1>

And you're trying to make sure now two pages can have the same key value?

[btrem]

Something like this, I imagine:

      if (ids.has(postKey)) {
        throw new Error(`Duplicate "${key}" found ("${postKey}"): ${inputPath} <=> ${ids.get(postKey)}`);
      }
      else {
          ids.set(postKey, inputPath);
      }

But that is not working. I haven't figured out the logic, but I think that's the right aim.

[pdehaan]

"But it does not work for a --serve script"

Yeah, I usually avoid using --serve, but i guess the cheap-n-easy hack is to just ignore duplicates if the current post.data.page.inputPath and cached inputPath are the same.

[pdehaan]

You shouldn't need that else because the throw will abort the current context anyways and launch you into the sun. 🚀🌞

[btrem]

Yeah, I was trying to avoid placing duplicates in the Map. While playing around, and console.logging the Map, I noticed that duplicates weren't being added. I'm new to Map, so I looked it up, and it appears that duplicates aren't added. So the solution is easier than I thought:

//...
if (ids.has(postKey) && inputPath != ids.get(postKey)) {
   throw new Error(`Duplicate "${key}" found ("${postKey}"): ${inputPath} <=> ${ids.get(postKey)}`);
}
//...

[pdehaan]

Yeah, not sure what the issue would be.
I did this before seeing your exact code above and it seemed to work for me after force quitting the --serve in VS Code and rerunning my eleventy --serve command and it didn't seem to throw errors when saving files and triggering the --serve rebuild.

      if (ids.has(postKey) && ids.get(postKey) !== inputPath) {
        throw new Error(`Duplicate "${key}" found ("${postKey}"): ${inputPath} <=> ${ids.get(postKey)}`);
      }